1. Field of the Invention
The present invention relates to air conditioning systems and more particularly relates to room air heating and cooling units having unit-mounted thermostatic controls.
2. Prior Art
Air conditioning units for heating and cooling room air under control of unit-mounted thermostats are generally known. One widespread application of such units is in hotel or motel rooms where individual units can be operated by the users of the room in which the units are located. These air conditioning units are generally known as "package terminal" units in that they can be installed in the room and used without requiring ducting or a separate thermostat to be installed in the walls of the room. Such units are highly desirable because their use minimizes the cost of buildings in which they are installed and simplifies repair and replacement of the units.
A typical package air terminal includes an air-circulating blower, an air-cooling device, an air-heating device and a thermostatic control for sensing the air temperature and governing operation of the air heating and cooling devices. In such a unit, the blower is operated to induce a flow of room air into a heat-exchange relationship with the heating and cooling devices after which the air, which has either been heated or cooled, is discharged back into the room through a suitable discharge vent. In some cases the cooling device is formed by the evaporator of a compressor-condenser-evaporator mechanical refrigeration system, while the air-heating device is formed by electric resistance heated elements. Other units employ so-called heat pump systems which are formed by compressor-condenser-evaporator refrigeration systems in which the flow of refrigerant in the system is reversable. The air heating and cooling devices are a single refrigerant coil which alternately functions as the evaporator or as the condenser or the refrigeration system depending upon the refrigerant flow direction.
The thermostatic controls for these units have typically been formed by a control unit including a switch arrangment for cutting-in and cutting-out the air-heating and air-cooling devices, and a thermostatic actuator which operates the switch arrangement in response to the sensed temperature of room air which returns to the unit.
Most package air terminal units are constructed so that room air which returns to the unit does so by traveling along the floor of the room. Air which is discharged from the unit is normally discharged upwardly into the space. The unit-mounted thermostats are normally constructed so that they sense the temperature of the air returning to the unit. This presents no serious problems in the cooling mode because the chilled air directed into the room by the unit tends to gravitate to the floor for return to the unit. When the unit operates to heat the room, however, the warm air discharged upwardly into the space does not gravitate to the floor and hot air can be directed into the space for a substantial period of time before warm air is returned to the unit from along the floor. As a result the room air temperature tends to fluctuate widely from desired set point temperatures and substantial overheating and energy consumption are experienced.
One prior art approach to solving the problem has been to place a heating anticipator adjacent the thermostatic sensor. Such anticipators are generally small heaters which supply heat to the thermostatic sensing element whenever the unit is in its heating mode. This tends to increase the cycling rate of the heater and moderate the room air temperature. Unfortunately, as the ratio of "on" to "off" time increases, the temperature sensor is more effected by the anticipator-heater than by the actual room temperature. The result of this is in effect known as temperature "droop" in which the average room temperature drops steadily downwards as the heating load in the room increases.
In certain circumstances a similar problem can be encountered when a room is being cooled. In particular when outdoor temperatures are low, or decrease rapidly, the discharge air temperature from the unit can become quite low compared to the set point temperature. The room thus tends to become subcooled before the unit air cooling equipment is cycled off. Providing a cooling anticipator of some sort is not practical because under normal operating conditions the air cooling equipment would tend to be short cycled.